Electrolytes and method for anodizing aluminum



United States Patent 3,390,063 ELECTROLYTES AND METHOD FOR ANGDIZING ALUMINUM Kenneth C. Working, 3409 E. Ocean Ave., Long Beach, Calif. 90807 No Drawing. Filed Dec. 28, 1964, Ser. No. 421,722 7 Claims. (Cl. 204-58) ABSTRACT OF THE DISCLOSURE Aluminum is anodized in an aqueous electrolyte con taining an anodizing acid and an additive made by stee ing redwood in water until a solution having a pH of about 2 to 5 is formed.

This invention relates to and has for its object the production of anodized aluminum oxide coatings which are of superior hardness. It is also related to and has for its object the processes for production of such objects.

It is Well known that aluminum and its alloys, herein collectively referred to as aluminum, can be electrolytically anodized employing anodizing electrolytes such as sulfuric acid, sulfamic acid, oxalic acid, chromic acid and phosphoric acid by passing current through an electrolytic cell of which the aluminum is an electrode employing electrolytes. The most common electrolyte employed is sulfuric acid. While temperatures up to 90 F. have been employed, increased wear resistance'is obtained when using sulfuric acid as an electrolyte at temperatures of about 30 to 40 F. or lower. While concentrations of 5% to 25% by volume have been recommended when using sulfuric acid, the usual concentration is in the neighborhood of about by volume of sulfuric acid, 66 B.

I have discovered that aqueous extracts of wood, described in my Patent 2,838,877, particularly when obtained from redwood, when added to the anodizing acid electrolytes herein referred to, in the above procedure, will produce harder coats than can be obtained with the same electrolytes under like conditions not employing the extract of my invention.

While I do not wish to be bound by any theory of why my additive produces superior results, I have observed that the additive reduces the rate of corrosive attack of the anodizing acid upon the aluminum. I have also observed that the oxide coats produced employing my additive, when mixed with sulfuric acid as the anodizing electrolyte, contain materially less sulfate ion than do coats produced employing the acid without the additive.

I have observed that the extract has a chelating action in that it will sequester metal ions, for example, iron. It is possible, although I do not Wish to be bound by this theory, that the property of the additive to produce harder coats may in part arise from chelating action of the additive.

Whatever the explanation, observation does show that I may produce harder coats by using my additive than may be produced by the same process without the additive of my invention.

As is well known in this art, the rate of production of oxide coating becomes appreciable when a voltage of about to volts is reached. Another critical range is in the region of volts. This is known as the critical voltage. Great care must be taken in the rate of increase of voltage in approaching and traversing this region, in order to avoid burning, that is, puncturing of the coating which is being deposited. The result of such puncture is an increase in the corrosive attack and a limitation of the build-up of coating at the burned region.

I have found that I may advance the voltage, and thus,

3,390,053 Patented June 25, 1968 the rate of build-up of the coat, through these regions without burning and employ a time rate of advance of voltage not usually found possible when using sulfuric acid electrolytes without my additive.

The additive of my invention may be made by extracting the wood, as described in my patent referred to above. Wood or bark in the form of chips or sawdust, may be steeped in water at temperatures from room temperature to temperatures above 212 F., using pressure cookers. The lower the temperature, the longer it takes to perform t e extraction and attain the low pH produced by the extracted acid. The pH may be in the region of about 2 to about 5 depending on the time taken in the extraction and the temperature. Preferably, I steep sawdust or chips of the redwood, herein collectively referred to as redwood, in water heated to a temperature in the range of about 180 F. to boiling at atmospheric pressure, i.e. 212 F. or heat to a more elevated temperature under superatmospheric pressure, and filter off the sawdust or chips. The additive may be added to the acid to produce the desired concentration of additive. About 1% to 30% by volume 66 B. sulfuric acid to the mixed electrolyte may be employed. Preferably, I employ from about 5 to 20 parts of volume of 66 Baum acid to make up parts by volume of the mixed water and additive.

To this dilute sulfuric acid I add a. small portion of the above extract, employing from about 2% to 20% by volume based on the mixed acid-water and additive.

The anodizing procedure employing the mixed electrolyte may be any of the processes of the prior art, such as are employed in producing hard coatings of the prior art. The voltage, current densities and rate of rise of voltage such as are employed in well known prior art processes for anodizing aluminum may be employed. The temperature employed is preferably in the range of above freezing to above 50 F., but preferably a temperature of about 35 F. is desirable. Temperatures in the range of 10 to 15 F. are also suitable. Agitation to produce uniform coats is preferably employed.

The conditions and procedures conventionally employed in anodizing employing sulfuric acid electrolytes may be followed. Thus, the voltage may be raised at the rate of 1 volt per minute for the time necessary to deposit the coat desired. Alternatively, a constant current density may be maintained by controlling the voltage application as the resistance of the cell increases due to deposit of aluminum oxide on the electrode.

The current densities which may be employed will vary over a wide range, depending upon the geometry of the system, the internal cell resistance and the total number of square feet of surface exposed for anodizing. United States Patent 2,692,851, issued to Charles F. Burrows, is referred to for a description of a sulfuric acid anodizing process and the conditions for operation, as illustrated in the prior art procedures. The procedures described in said patent and other of those conventionally previously employed in the prior art may be employed in operating in the processes of my invention.

The improvement obtained by employing the additive of my invention in such procedures may be further described by the following examples, which are given only to illustrate the advantages obtained by employing the additive of my invention and not as a limitation thereof.

Example 1 Sawdust from redwood was placed in a pressure cooker :and water sufficient to coat the sawdust and just suflicient to leave a water surface over the sawdust was added. The mixture was heated to a temperature of about 211 to about 213 F. for minutes. It was then cooled to ambient temperature and filtered. The extract had a specific gravity of 1.003 and a pH of 3.5. On evaporation to dryness, the residue produced was equal to 1.18% by weight of the original filtrate.

Example 2 A solution was made containing by volume of 66 B. sulfuric acid in water. This solution is hereinafter referred to as Electrolyte #1.

A solution was made up containing 12.5 by volume of the above extract (Example 1), and the remainder of sulfuric acid solution containing 16% by volume of 66 B. sulfuric acid, based upon the mixture of acid, water and the above extract volume. This solution is hereinfater referred to as Electrolyte #2.

Panels composed of A x 4" x 4" of 2024 T-3 aluminum were anodized employing direct current, the voltage being increased at the rate of one volt per minute from a voltage of volts to 60 volts. The temperature was kept at 31 F., with constant agitation. The coating thickness of the panels before and after anodizing was measured at 8 stations. The panels were then subjected to Taber abrasion test. This test is a standard pr ocedure employed in this art, and is described in the above Burrows patent. The test consists of rotating two standard abrasive wheels which are caused to track on the surface of the test panel with a predetermined load. The wheels notate on their axles, and the wheels individually rotate with their axis at an angle to the axis of the specimen, driven by the specimen. Each revolution about the center is termed a cycle.

The panels were measured as to thickness at each of 8 stations before submission to the Taber test.

The following tests were made employing 10,000 cycles.

The coating thickness loss from the coat produced employing Electrolyte #1 was 5.3 10" inches, and employing Electrolyte #2 was 3.8 10 inches as an average of the loss over the 8 stations.

The same test was repeated after 40,000 cycles.

The panel coated with Electrolyte #1 showed an average coating loss of 11 10- inches, whereas that employing Electrolyte #2, show an average loss of 8.7 X 10 inches.

It is apparent from these tests that the coat produced using the additive was considerably more wear-resistant than the coat produced using the same acid under the same conditions without the additive.

Example 3 An additive was produced according to Example 1. The specific gravity of the extract was 1.003 and the pH was 4. The panels, each 16 B & S gage x 1" x 6", were anodized using Electnolyte #1, that is, 16% sulfuric acid in water without additive, and with an additive composed of 12.5% of the extract described above, and water, and containing 16% by volume of sulfuric acid, 66 B. based upon the mixture of acid, water and additive. This electrolyte was termed Electrolyte #3.

The above panels, each of 2024-T3 aluminum, were anodized by the above procedure, voltage being increased at the rate of one volt per minute, maintaining the temperature at 31 F., until each panel had a coating thickness of 0.002". Each of the panels was then stripped of its oxide coating to remove all the oxide from the coated panel. The dissolved coating was analyzed for sulfate precipitated as barium sulfate.

The precipitates weighed as follows:

Precipitate from the panel of Electrolyte #1 was .08 mg.

Precipitate from the panel of Electrolyte #3 was .04 mg.

This indicates that the sulfate ion inclusion was half as much using the Electrolyte #3 as when Electrolyte #1 is employed.

The sulfate ion comes from the absorption of sulfuric acid and aluminum sulfate from the solution.

The effect of the lesser quantity of included acidic material in the panels employing the process of my invention may explain the comparative hardness of the two coatings. The included sulfate ion, it is believed, has a softening action and the higher the concentration of sulfate ion, the greater is this softening action.

Example 4 Solution- (1) Water 84%, 66 Sulfuric Acid 16%,

(2) Water 78%, 66 Sulfuric Acid 16%, Example 1 Additive 6%,

(3) Water 71%, 66 Sulfuric Acid 16%, Example 1 Additive 12.5%,

(4) Water 59%, 66 Sulfuric Acid 16%, Example 1 Additive 25%.

Four specimens of 1100 Aluminum, all of the same size were completely immersed in the solutions, one in each solution. Table I shows the loss in weight of each specimen at the duration of time listed.

TAB LE I Weight Loss in Grams Solution 200 Hours 300 Hours 600 Hours 1,200 Hours Example 5 Two more solutions were prepared, the same as #1 and #2 above. Two specimens of the same dimensions of 2024 T3 Aluminum were completely immersed in solutions 1 and 2 of Example 4.

Table II shows the loss of weight of these specimens at the time durations listed.

TABLE II Weight Loss in Grams Solution Hours 250 Hours 350 Eours Example 6 A specimen of 1100 Aluminum was subjected to Additive No. 1 of Example 1, at the boiling point of water for 50 hours. It showed the same weight after treatment as it had before treatment.

It will be observed that the corrosivity of the extract is substantially less than the corrosivity of the sulfuric acid solution.

The lower corrosivity of the acid solution, taken to gether with the reduced inclusion of sulfate ions, may be an additional reason for the increased hardness produced by this process.

A further property of the extract of my invention is its chelating action on metal ions, as is illustrated by the sequestering action of the extract on the iron ion. To illustrate this property, the following is given by way of example.

Example 7 The redwood previously referred to was extracted according to Example 1, but employing in this case, an iron kettle, whereas in Example 1 a porcelain-lined vessel was employed. The extract so produced was employed in the same proportions as Electrolyte #2, of Example 2, and is herein referred to as Electrolyte #3.

The extract produced as above was further treated by reacting the extract with powdered iron in excess until no more iron was taken up by the extract. The iron was filtered off, and a similar solution containing the same proportions as previously described for Electrolyte #3 was made. This is referred to as Electrolyte #4.

Both of the extracts employed to produce Electrolyte #3 and Electrolyte #4 showed no iron reaction with potassium ferrocyanide, indicating, according to this test, no presence of ionic iron in solution. However, when these extracts were evaporated to dryness and incinerated in an iron-free apparatus, the ash was red; and when taken up in hydrochloric acid with a small amount of nitric acid, and after dilution with water, a strong blue color developed using potassium ferrocyanide, indicating that the iron was present in a non-ionic form in the extract. This indicates the chelating action of the extract.

Electrolyte #3 and Electrolyte #4 were employed in anodizing aluminum according to the procedure of Example 2. Employing the solution saturated with iron, a coat of 15.5 inches was produced in 55 minutes, employing a cumulative current of 429 ampere minutes, while the solution of Electrolyte #4 produced a coat of 10 inches in 63 minutes employing 446 ampere minutes. It thus appears that the chelated electrolyte has similar activity to the unchelated electrolyte of Example 1.

It is to be recognized that neutralized extract such as referred to in my Patent 2,838,377 may be employed, since, in the acid solution, the acidified neutralized extract will be converted by either precipitation of the cation or its release in the neutralization of the cation by the acid of the electrolyte.

While I have described particular embodments of my invention for the purpose of illustration, it should be understood that various modifications and adaptations thereof may be made within the spirit of the invention, as set forth in the appended claims.

I claim:

1. The proces of anodizing aluminum which comprises subjecting said aluminum as anode to electrolytic action in an electrolytic cell of which the electrolyte is a water solution comprising a mixture of anodizing acid and an additive produced by steeping redwood in water at temperatures from room temperature to the boiling point of water for a time sutlicient to produce an additive having a pH of about 2 to 5 and separating said redwood from said additive so formed, said additive being present in an amount sufiicient to produce a hard oxide coating on said aluminum.

2. The process defined in claim 1 wherein said anodizing acid is sulfuric acid.

3. The process of claim 2, in which the sulfuric acid is 66- Baurn acid from about 1% to about by volume of the electrolyte mixture.

4. The process of claim 1, in which the said additive is of from about 2 to about 20% by volume of the electrolyte mixture.

5. An aqueous anodizing electrolyte for the anodic oxidation of aluminum consisting essentially of water, an anodizing acid and an additive produced by steeping redwood in water at temperatures from room temperature to the boiling point of water for a time sufiicient to produce an additive having a pH of about 2 to 5 and separating said redwood from said additive so formed, said additive being present in an amount sufiicient to produce a hard oxide coating on said aluminum.

6. The electrolyte of claim 5, in which the acid is sulfuric acid.

7. The electrolyte of claim 5, in which said additive is from about 2% to about 20% by volume of the electrolyte.

References Cited UNITED STATES PATENTS 2,743,221 4/ 1956 Sanford 204-58 2,897,125 7/1959 Franklin 204-58 3,146,178 8/1964 Cochran et a1. 204-58 JOHN H. MACK, Primary Examiner.

HOWARD S. WILLIAMS, Examiner.

G. KAPLAN, Assistant Examiner. 

